334 research outputs found
Gaia FGK Benchmark Stars: Effective temperatures and surface gravities
Large Galactic stellar surveys and new generations of stellar atmosphere
models and spectral line formation computations need to be subjected to careful
calibration and validation and to benchmark tests. We focus on cool stars and
aim at establishing a sample of 34 Gaia FGK Benchmark Stars with a range of
different metallicities. The goal was to determine the effective temperature
and the surface gravity independently from spectroscopy and atmospheric models
as far as possible. Fundamental determinations of Teff and logg were obtained
in a systematic way from a compilation of angular diameter measurements and
bolometric fluxes, and from a homogeneous mass determination based on stellar
evolution models. The derived parameters were compared to recent spectroscopic
and photometric determinations and to gravity estimates based on seismic data.
Most of the adopted diameter measurements have formal uncertainties around 1%,
which translate into uncertainties in effective temperature of 0.5%. The
measurements of bolometric flux seem to be accurate to 5% or better, which
contributes about 1% or less to the uncertainties in effective temperature. The
comparisons of parameter determinations with the literature show in general
good agreements with a few exceptions, most notably for the coolest stars and
for metal-poor stars. The sample consists of 29 FGK-type stars and 5 M giants.
Among the FGK stars, 21 have reliable parameters suitable for testing,
validation, or calibration purposes. For four stars, future adjustments of the
fundamental Teff are required, and for five stars the logg determination needs
to be improved. Future extensions of the sample of Gaia FGK Benchmark Stars are
required to fill gaps in parameter space, and we include a list of suggested
candidates.Comment: Accepted by A&A; 34 pages (printer format), 14 tables, 13 figures;
language correcte
Signatures of rocky planet engulfment in HAT-P-4. Implications for chemical tagging studies
Aims. To explore the possible chemical signature of planet formation in the
binary system HAT-P-4, by studying abundance vs condensation temperature Tc
trends. The star HAT-P-4 hosts a planet detected by transits while its stellar
companion does not have any detected planet. We also study the Lithium content,
which could shed light on the problem of Li depletion in exoplanet host stars.
Conclusions. The exoplanet host star HAT-P-4 is found to be ~0.1 dex more metal
rich than its companion, which is one of the highest differences in metallicity
observed in similar systems. This could have important implications for
chemical tagging studies, disentangling groups of stars with a common origin.
We rule out a possible peculiar composition for each star as lambda Boo, delta
Scuti or a Blue Straggler. The star HAT-P-4 is enhanced in refractory elements
relative to volatile when compared to its stellar companion. Notably, the
Lithium abundance in HAT-P-4 is greater than in its companion by ~0.3 dex,
which is contrary to the model that explains the Lithium depletion by the
presence of planets. We propose a scenario where, at the time of planet
formation, the star HAT-P-4 locked the inner refractory material in
planetesimals and rocky planets, and formed the outer gas giant planet at a
greater distance. The refractories were then accreted onto the star, possibly
due to the migration of the giant planet. This explains the higher metallicity,
the higher Lithium content, and the negative Tc trend detected. A similar
scenario was recently proposed for the solar twin star HIP 68468, which is in
some aspects similar to HAT-P-4. We estimate a mass of at least Mrock ~ 10
Mearth locked in refractory material in order to reproduce the observed Tc
trends and metallicity.Comment: 5 pages, 6 figures, A&A Letters accepte
Stellar twins determine the distance of the Pleiades
© 2016 ESO.Since the release of the Hipparcos catalogue in 1997, the distance to the Pleiades open cluster has been heavily debated. The distance obtained from Hipparcos and those by alternative methods differ by 10 to 15%. As accurate stellar distances are key to understanding stellar structure and evolution, this dilemma puts the validity of some stellar evolution models into question. Using our model-independent method to determine parallaxes based on twin stars, we report individual parallaxes of 15 FGK type stars in the Pleiades in anticipation of the astrometric mission Gaia. These parallaxes give a mean cluster parallax of 7.42 ± 0.09 mas,which corresponds to a mean cluster distance of 134.8 ± 1.7 pc. This value agrees with the current results obtained from stellar evolution models
Stellar parameters and chemical abundances of 223 evolved stars with and without planets
We present fundamental stellar parameters and chemical abundances for a
sample of 86 evolved stars with planets and for a control sample of 137 stars
without planets. The analysis was based on both high S/N and resolution echelle
spectra. The goals of this work are i) to investigate chemical differences
between stars with and without planets; ii) to explore potential differences
between the properties of the planets around giants and subgiants; and iii) to
search for possible correlations between these properties and the chemical
abundances of their host stars. In agreement with previous studies, we find
that subgiants with planets are, on average, more metal-rich than subgiants
without planets by ~ 0.16 dex. The [Fe/H] distribution of giants with planets
is centered at slightly subsolar metallicities and there is no metallicity
enhancement relative to the [Fe/H] distribution of giants without planets.
Furthermore, contrary to recent results, we do not find any clear difference
between the metallicity distributions of stars with and without planets for
giants with M > 1.5 Msun. With regard to the other chemical elements, the
analysis of the [X/Fe] distributions shows differences between giants with and
without planets for some elements, particularly V, Co, and Ba. Analyzing the
planet properties, some interesting trends might be emerging: i) multi-planet
systems around evolved stars show a slight metallicity enhancement compared
with single-planet systems; ii) planets with a 0.5 AU orbit
subgiants with [Fe/H] > 0 and giants hosting planets with a 1 AU
have [Fe/H] < 0; iii) higher-mass planets tend to orbit more metal-poor giants
with M < 1.5 Msun, whereas planets around subgiants seem to follow the
planet-mass metallicity trend observed on dwarf hosts; iv) planets orbiting
giants show lower orbital eccentricities than those orbiting subgiants and
dwarfs.Comment: 49 pages, 31 figures, Accepted for publication in A&A, abstract
shortened - corrected references, typos, acknowledgements include
Constraints on temporal variation of fundamental constants from GRBs
The formation of a strange or hybrid star from a neutron star progenitor is
believed to occur when the central stellar density exceeds a critical value. If
the transition from hadron to quark matter is of first order, the event has to
release a huge amount of energy in a very short time and we would be able to
observe the phenomenon even if it is at cosmological distance far from us; most
likely, such violent quark deconfinement would be associated with at least a
fraction of the observed gamma ray bursts. If we allow for temporal variations
of fundamental constants like or , we can expect that
neutron stars with an initial central density just below the critical value can
enter into the region where strange or hybrid stars are the true ground state.
From the observed rate of long gamma ray bursts, we are able to deduce the
constraint , which is about 5
orders of magnitude more stringent than the strongest previous bounds on a
possible increasing .Comment: 5 pages, no figure. v3: refereed versio
The Gaia-ESO Survey: Separating disk chemical substructures with cluster models
(Abridged) Recent spectroscopic surveys have begun to explore the Galactic
disk system outside the solar neighborhood on the basis of large data samples.
In this way, they provide valuable information for testing spatial and temporal
variations of disk structure kinematics and chemical evolution. We used a
Gaussian mixture model algorithm, as a rigurous mathematical approach, to
separate in the [Mg/Fe] vs. [Fe/H] plane a clean disk star subsample from the
Gaia-ESO survey internal data release 2. We find that the sample is separated
into five groups associated with major Galactic components; the metal-rich end
of the halo, the thick disk, and three subgroups for the thin disk sequence.
This is confirmed with a sample of red clump stars from the Apache Point
Observatory Galactic Evolution Experiment (APOGEE) survey. The two
metal-intermediate and metal-rich groups of the thin disk decomposition
([Fe/H]>-0.25 dex) highlight a change in the slope at solar metallicity. This
holds true at different radial regions. The distribution of Galactocentric
radial distances of the metal-poor part of the thin disk ([Fe/H]<-0.25 dex) is
shifted to larger distances than those of the more metal-rich parts. Moreover,
the metal-poor part of the thin disk presents indications of a scale height
intermediate between those of the thick and the rest of the thin disk, and it
displays higher azimuthal velocities than the latter. These stars might have
formed and evolved in parallel and/or dissociated from the inside-out formation
taking place in the internal thin disk. Their enhancement levels might be due
to their origin from gas pre-enriched by outflows from the thick disk or the
inner halo. The smooth trends of their properties (their spatial distribution
with respect to the plane, in particular) with [Fe/H] and [Mg/Fe] suggested by
the data indicates a quiet dynamical evolution, with no relevant merger events
The Gaia-ESO Survey: Detailed Abundances in the Metal-poor Globular Cluster NGC 4372
We present the abundance analysis for a sample of 7 red giant branch stars in
the metal-poor globular cluster NGC 4372 based on UVES spectra acquired as part
of the Gaia-ESO Survey. This is the first extensive study of this cluster from
high resolution spectroscopy. We derive abundances of O, Na, Mg, Al, Si, Ca,
Sc, Ti, Fe, Cr, Ni, Y, Ba, and La. We find a metallicity of [Fe/H] = -2.19
0.03 and find no evidence for a metallicity spread. This metallicity
makes NGC 4372 one of the most metal-poor galactic globular clusters. We also
find an {\alpha}-enhancement typical of halo globular clusters at this
metallicity. Significant spreads are observed in the abundances of light
elements. In particular we find a Na-O anti-correlation. Abundances of O are
relatively high compared with other globular clusters. This could indicate that
NGC 4372 was formed in an environment with high O for its metallicity. A Mg-Al
spread is also present which spans a range of more than 0.5 dex in Al
abundances. Na is correlated with Al and Mg abundances at a lower significance
level. This pattern suggests that the Mg-Al burning cycle is active. This
behavior can also be seen in giant stars of other massive, metal-poor clusters.
A relation between light and heavy s-process elements has been identified.Comment: 14 pages, 14 figures, accepted for publication in A&
The Gaia-ESO Survey: the chemical structure of the Galactic discs from the first internal data release
Most high-resolution spectroscopic studies of the Galactic discs were mostly
confined to objects in the solar vicinity. Here we aim at enlarging the volume
in which individual chemical abundances are used to characterise both discs,
using the first internal data release of the Gaia-ESO survey. We derive and
discuss the abundances of eight elements (Mg, Al, Si, Ca, Ti, Fe, Cr, Ni, and
Y). The trends of these elemental abundances with iron are very similar to
those in the solar neighbourhood. We find a natural division between alpha-rich
and alpha-poor stars, best seen in the bimodality of the [Mg/M] distributions
in bins of metallicity, which we attribute to thick- and thin-disc sequences,
respectively. With the possible exception of Al, the observed dispersion around
the trends is well described by the expected errors, leaving little room for
astrophysical dispersion. Using previously derived distances from Recio-Blanco
et al. (2014b), we further find that the thick-disc is more extended vertically
and is more centrally concentrated towards the inner Galaxy than the thin-disc,
which indicates a shorter scale-length. We derive the radial and vertical
gradients in metallicity, iron, four alpha-element abundances, and Al for the
two populations, taking into account the identified correlation between R_GC
and |Z|. Radial metallicity gradient is found in the thin disc. The positive
radial individual [alpha/M] gradients found are at variance from the gradients
observed in the RAVE survey. The thin disc also hosts a negative vertical
metallicity gradient, accompanied by positive individual [alpha/M] and [Al/M]
gradients. The thick-disc, presents no radial metallicity gradient, a shallower
vertical metallicity gradient than the thin-disc, an alpha-elements-to-iron
radial gradient in the opposite sense than that of the thin disc, and positive
vertical individual [alpha/M] and [Al/M] gradients.Comment: 24 pages, 10 figure
Elemental abundances differences in the massive planet-hosting wide binary HD 196067-68
It has been suggested that small chemical anomalies observed in
planet-hosting wide binary systems could be due to planet signatures, where the
role of the planetary mass is still unknown. We search for a possible planet
signature by analyzing the Tc trends in the remarkable binary system
HD196067-HD196068. At the moment, only HD196067 is known to host a planet which
is near the brown dwarf regime. We take advantage of the strong physical
similarity between both stars, which is crucial to achieving the highest
possible precision in stellar parameters and elemental chemical abundances.
This system gives us a unique opportunity to explore if a possible depletion of
refractories in a binary system could be inhibited by the presence of a massive
planet. We performed a line-by-line chemical differential study, employing the
non-solar-scaled opacities, in order to reach the highest precision in the
calculations. After differentially comparing both stars, HD196067 displays a
clear deficiency in refractory elements in the Tc plane, a lower iron content
(0.051 dex) and also a lower Li I content (0.14 dex) than its companion. In
addition, the differential abundances reveal a Tc trend. These targets
represent the first cases of an abundance difference around a binary system
hosting a super-Jupiter. Although we explored several scenarios to explain the
chemical anomalies, none of them can be entirely ruled out. Additional
monitoring of the system as well as studies of larger sample of wide binary
systems hosting massive planets, are needed to better understand the chemical
abundance trend observed in HD196067-68.Comment: 9 pages, six figures, three table
Gaia FGK Benchmark stars: Opening the black box of stellar element abundance determination
Gaia and its complementary spectroscopic surveys combined will yield the most comprehensive database of kinematic and chemical information of stars in the Milky Way. The Gaia FGK benchmark stars play a central role in this matter as they are calibration pillars for the atmospheric parameters and chemical abundances for various surveys. The spectroscopic analyses of the benchmark stars are done by combining different methods, and the results will be affected by the systematic uncertainties inherent in each method. In this paper, we explore some of these systematic uncertainties. We determined line abundances of Ca, Cr, Mn and Co for four benchmark stars using six different methods. We changed the default input parameters of the different codes in a systematic way and found, in some cases, significant differences between the results. Since there is no consensus on the correct values for many of these default parameters, we urge the community to raise discussions towards standard input parameters that could alleviate the difference in abundances obtained by different methods. In this work, we provide quantitative estimates of uncertainties in elemental abundances due to the effect of differing technical assumptions in spectrum modelling
- âŠ